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1. Field of the Invention
The present invention relates to methods for high-resolution compression of images, and more particularly to methods for dynamic imaging of operating faults in integrated circuits in which compressed, high-resolution icon images are created from larger images representing operating states of integrated circuits under test.
2. Description of Related Art
Decreases in the size of internal features of Very Large Scale Integrated (VLSI) circuit devices demand even faster and more reliable design and testing. Conventional Integrated Circuit (IC) testers can only retrieve information from the external pins of the Device Under Test (DUT), thus limiting subsequent diagnoses. A failure detected with a conventional tester may be caused by a discrepancy at any point inside the component. If the DUT has hundreds of thousands of gates, fault identification becomes a complex and tedious chore.
In a typical test operation, a conventional IC tester (one which applies stimuli to input pins of the IC and measures the results at the IC output pins) detects a fault at some vector (for example, vector v) in a test vector sequence when testing an IC device. The test sequence may contain a large number of test vectors, for example, 100 or more test vectors, each vector representing a set of stimuli, such as input voltages, applied to the pins of the DUT. The origin of the detected fault occurs somewhere in the chip, at some vector (for example, vector a), between the first vector of the sequence (vector 1) and the vector at which the fault was detected (vector v). This fault then propagates forward and appears at an external pin or bond pad of the DUT at vector v. It is then desired to identify the nature, location and time of generation of the fault occurring at vector a.
Internal probing of the chip is therefore necessary. For many years, the preferred solution has been to prove the chip using a low energy (around 1 keV) scanning electron microscope (SEM). See, for example, E. Menzel & E. Kubalek, Fundamentals of Electron Beam Testing of Integrated Circuits, 5 Scanning 103-122 (1983), and E. Plies & J. Otto, Voltage Measurement Inside Integrated Circuit Using Mechanical and Electron Probes, IV SCANNING ELECTRON MICROSCOPY 1491-1500 (1985).
Until recently, the SEM was a sophisticated lab instrument, used only by experienced researchers. In 1987, the "IDS 5000.TM." workstation-based, electron-beam test system was commercially introduced by Schlumberger. S. Concina, G. Liu, L. Lattanzi, S. Reyfman & N. Richardson, Software Integration in a Workstation Based E-Beam Tester, International Test Conference Proceedings (1986); N. Richardson, E-Beam Probing for VLSI Circuit Debug, VLSI Systems Design (1987); S. Concina & N. Richardson IDS 5000: an Integrated Diagnosis System for VLSI, 7 Microelectronic Engineering (1987). See also U.S. Pat. Nos. 4,706,019 and 4,721,909 to N. Richardson.
Measurement and information gathering is thus no longer a major problem in analyzing IC failures. Organizing the extensive and detailed information obtained about an IC with such a tester is, however, critical to rapid and effective fault diagnosis.
Rather than seeking the answer to an absolute problem ("Why does this device fail?"), it may be preferable to address a relative problem ("Where, when and why does this device behave differently from a known good device?"). With the latter approach to diagnosis, operating faults in the IC can be traced.
To perform such a diagnosis, a time period of interest between 0 (the beginning of the test sequence of interest) and the first fault detection is selected. This period is divided into intervals such that, in each interval, the IC has a fixed behavior. Each interval thus corresponds to an operating state of the IC. After the states of interest have been identified, a comparison can be made between a known "good" IC device and the DUT, at each state.
Such comparisons can be made with stroboscopic voltage contrast images. The device under test (DUT) is stimulated with a sequence of test "vectors" in a conventional E-Beam test system such as the Schlumberger "IDS 5000." Each test "vector" represents a specified set of stimuli, such as input voltages applied to the pins of the IC. During the application of each vector of the sequence, the DUT is in a state for a period of time called a "strobe window". By pulsing the electron beam repeatedly in a certain phase to the beginning of the sequence, a stroboscopic image representing the state of the DUT in any desired strobe window can be obtained. This strobe process is analogous to that of using a stroboscopic light to "freeze" the operation of an automobile engine to adjust ignition timing. A series of state images makes up a stack.
The process is repeated using a known good IC device (also called a golden die), so as to acquire a stack of stroboscopic images representing its states in response to the same series of test vectors. Each of the images may, for example, be a graphical representation in digital format in the form of a 512.times.512 matrix of pixels of varying intensity, the intensities being represented by a value between 0 and 255 (an 8-bit value). The images thus acquired, representing operating states of the DUT, can be stored and used to diagnose operating faults in the DUT.
After the images have been acquired, the stacks may be compared. The comparison may take the form of subtracting an image of the golden die from an image of the DUT (or vice versa), pixel by pixel, where the compared images are those produced in response to the same set of stimuli. The comparison is repeated, image by image, so as to produce a stack of "difference" images.
Alternatively, the two stacks may represent states of a single DUT in response to two different sets of stimuli. For example, a DUT may operate as designed at a given temperature or with a given input voltage, but fail at higher temperature or input voltage. One stack of images may be acquired representing correct operation of the DUT under one set of circumstances and a second stack acquired representing failed operation under a second set of circumstances. These stacks may also be compared, image by image, to produce a stack of "difference" images.
If the good and faulty devices behave the same way, the images are the same. Likewise, if a device subjected to different sets of stimuli is operating the same way in response to the different sets of stimuli, the images are the same. Any divergence between the two sets of images may be considered as a discrepancy in the test (or failing) device. T. May, G. Scott, E. Meieran, P. Wiener & V. Rao, Dynamic Fault Imaging of VLSI Random Logic Devices, INTERNATIONAL PHYSICS SYMPOSIUM PROCEEDINGS (1984). On a one-image-per-state basis, the comparison process reveals the propagation of the fault from its origin to the place where is first detected. The fault may appear as a discrepancy in the difference images; for example, a black or white line in an otherwise gray difference image may represent an incorrect logic level in the DUT. Such a comparison process is known as "Dynamic Fault Imaging" (DFI).
Of course, for the comparison to work properly, the two stacks of images must represent identical conditions: the same area of the chip, imaged at the same magnification under the same SEM operating conditions. However, it is possible that the images of one stack are tilted or rotated with respect to those of another stack due to slightly different orientation of the chips with respect to the SEM when the images are acquired. When perfect alignment is not possible, a spatial "warping" operation may need to be performed on each image of one stack to align it with the counterpart images of the other stack.
Once the images have been acquired, filtered, aligned and compared pixel-by-pixel to create a stack of resulting "difference" images, the difference images may be displaced to permit tracking of fault propagation through the series of images. The difference images may also be processed to enhance only the important information (the fault propagation) for analysis. However, for diagnostic purposes, information may be in over-abundance with some 512.times.512 pixels per image and 8 bits per pixel. The information needed for diagnosis is really only that which tracks fault propagation through the series of images.
When the images have been properly processed and the subtraction well-performed, the real DFI diagnosis can begin. Clues to the nature, location and time of origin of the fault are to be found somewhere in the stack of difference images. To find these clues efficiently, it is important to be able to access, display and visually compare the images quickly and interactively (in tenths of seconds). One way to do this would be to simultaneously display all images of the stack in juxtaposition, making the fault propagation readily visible. However, if each image comprises 512.times.512 pixels, only four images at a time can be displayed on a conventional high-resolution computer workstation display screen.
One way to display large numbers of images simultaneously on the screen (for easy visual tracking of the fault propagation) is to reduce the size of each image to form an "icon", or compressed version of the image, to be displayed on the screen. If, for instance, a 512.times.512 pixel image is compressed into a 64.times.64 pixel icon, each pixel of the icon represents a square of 8.times.8 pixels of the initial image. Using such icons, tens of images may be displayed simultaneously on the display screen. However, when the image is compressed into an icon, information is lost (in the example given above, 63/64 of the information from the initial image may be lost). Because the details of the icons are needed for DFI analysis, a high-resolution compression of the images is required. However, as many icon images are to be displayed at once, efficient DFI analysis demands that image compression be performed rapidly once the images to be displayed have been selected from a stack. To achieve "interactive" operation, it is desirable to carry out any selected process, such as preparation of an icon from an image, and display the result within, for example, a period of not more than two seconds.
Systems are known in the art which employ convolutions to perform geometric operation, filtering and pattern recognition. Such systems sometimes use fast Fourier transform to speed up the computation of large convolutions. Floating-point arithmetic is often used since it is not known in advance how many operations will be performed on the data. But with the use of floating point arithmetic, intermediate results are often more precise than necessary and therefore excessively time-consuming to compute. When this precision is not required, it results in a waste of time and resources.
Another way to speed up image processing is to employ dedicated processors designed to perform specific tasks at high speed. The cost of such processor can, however, add significantly to the cost of a test system. It is preferred instead to use the general-purpose processor of a standard engineering workstation to perform image processing, but without loss of processing time.
A principal aim of the present invention is to provide methods and apparatus for high resolution image compression in a workstation of the type provided in a system such as the Schlumberger "IDS 5000" with sufficient speed to permit Dynamic Fault Imaging with an interactive "feel", but without the need for dedicated hardware to perform the processing.
More broadly, it is an object of the present invention to provide methods and apparatus for high-resolution compression of images into icons.
It is a further object of the present invention to perform such compression such that a plurality of such icons may be simultaneously displayed, as an aid in DFI analysis of integrated circuits.
Still another object of the present invention is to provide for rapid and interactive compression of voltage contrast images in a general-purpose workstation associated with an electron-beam test probe system, without the need for special-purpose computing hardware to perform the compression.
Yet another object of the present invention is to permit two-dimensional emulation on a workstation display screen of a three-dimensional representation of a stack of images known as a state cube.